The present invention relates to an electrochemical gas sensor for determining the concentration of gas components in gas mixtures, in particular for determining the concentration of NOx and HC according to the preamble of independent claims 1 and 13.
European Patent Application 678 740 A1 describes a gas sensor of the aforementioned type for determining the NOx concentration in a gas mixture, in which two measuring gas spaces, each containing a pump cell, are arranged next to one another in a layer plane of a planar, oxygen ion conducting ceramic substrate. The measuring gas flows through a first diffusion opening into the first measuring gas space in which a first internal pump electrode is arranged. An external pump electrode is directly exposed to the measuring gas. The first internal pump electrode and the external pump electrode form the first pump cell. A predetermined oxygen partial pressure is set in the first measuring gas space via the first pump cell by pumping oxygen in and out. A concentration cell (Nernst cell) has a measuring electrode and a reference electrode connected to the atmosphere; the measuring electrode is arranged in the first measuring gas space. In order to set a constant oxygen partial pressure in the first measuring gas space, the voltage (electromotive force) of the concentration cell is controlled at a constant value via a pump voltage of the first pump cell. The first and second measuring gas spaces are connected by a connecting channel, which represents another diffusion opening; the atmosphere set at a constant oxygen partial pressure diffuses into the second measuring gas space via the connecting channel. Another internal pump electrode, which works together with the reference electrode arranged in the atmosphere reference channel and forms the second pump cell, is arranged in the second measuring gas space. The second internal pump electrode is made of a material, for example, rhodium, which reduces NO to N2 and O2. The reduced oxygen occurring at the second internal pump electrode is pumped, via a voltage applied to the pump, to the reference electrode and there it is released into the atmosphere. Since the atmosphere in the first measuring gas space is held at a constant oxygen partial pressure, the pump current for removing the reduced oxygen from the second measuring gas space is proportional to the NOx concentration. The measuring gas spaces and pump cells arranged in series are kept at different temperatures, the temperature at the electrodes in the first measuring gas space being set lower than the temperature at the electrode in the second measuring gas space. The sensor element design is relatively complicated and is only suitable for determining NOx.
The gas sensor according to the present invention having the characterizing features of claim 1 has the advantage that a basic sensor routinely manufactured for determining the lambda value of gas mixtures is used, to which only at least one further solid electrolyte layer having two additional electrodes has to be added. A sensor known as broad-band sensor, having a pump cell containing an internal and external pump electrode and a concentration cell containing a measuring electrode and a reference electrode, is used as the basic sensor. The measuring electrode and the internal pump electrode are arranged in a gas space opposite one another. In the sensor element according to the present invention this gas space forms the second measuring gas space. The use of a routinely manufactured basic lambda sensor offers considerable cost advantages compared to a sensor element design that is specialized for each application.
Another aspect of the present invention is, according to the characterizing features of claim 8, that the gas sensor can be used both as an NOx sensor and as an HC sensor with a single sensor element design. Only the taps at the electrode terminals are to be selected and the analysis circuit is to be adapted accordingly for either application. When the gas sensor is used as an HC sensor, the first electrode arranged in the first measuring gas space is used as a mixed potential electrode. When the gas sensor is used as an NOx sensor, the third electrode arranged in the second measuring gas space is used as an NOx reducing electrode. Additional cost advantages result through the wide application of the gas sensor.
The measures described in the subclaims allow advantageous refinements of and improvements on the sensor element presented in the main claim. The arrangement of two gas inlet openings symmetric with respect to the connecting channel ensures sufficient gas exchange of the measuring gas with the first measuring chamber and thus a short response time. In addition, this allows higher pump currents for the first pump cell.